linux-kernel - Re: [PATCH v4 1/1] mfd: Add I2C based System Configuaration (SYSCON) access

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Message-ID: <5d1d41504172d86d395b0135923f6f02@walle.cc>
Date:   Wed, 01 Jul 2020 23:01:26 +0200
From:   Michael Walle <michael@...le.cc>
To:     Lee Jones <lee.jones@...aro.org>
Cc:     robh+dt@...nel.org, broonie@...nel.org, gregkh@...uxfoundation.org,
        andriy.shevchenko@...ux.intel.com, devicetree@...r.kernel.org,
        linus.walleij@...aro.org, bgolaszewski@...libre.com, arnd@...db.de,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/1] mfd: Add I2C based System Configuaration (SYSCON)
 access

Am 2020-07-01 09:04, schrieb Lee Jones:
> On Wed, 01 Jul 2020, Michael Walle wrote:
> 
>> Hi Lee,
>> 
>> Am 2020-06-30 11:16, schrieb Michael Walle:
>> > I'm just trying to use this for my sl28 driver. Some remarks, see below.
>> >
>> > Am 2020-06-22 09:51, schrieb Lee Jones:
>> > > The existing SYSCON implementation only supports MMIO (memory mapped)
>> > > accesses, facilitated by Regmap.  This extends support for registers
>> > > held behind I2C busses.
>> > >
>> > > Signed-off-by: Lee Jones <lee.jones@...aro.org>
>> > > ---
>> > > Changelog:
>> > >
>> > > v3 => v4
>> > >   - Add ability to provide a non-default Regmap configuration
>> > >
>> > > v2 => v3
>> > >   - Change 'is CONFIG' present check to include loadable modules
>> > >     - s/#ifdef CONFIG_MFD_SYSCON_I2C/#if
>> > > IS_ENABLED(CONFIG_MFD_SYSCON_I2C)/
>> > >
>> > > v1 => v2
>> > >   - Remove legacy references to OF
>> > >   - Allow building as a module (fixes h8300 0-day issue)
>> > >
>> > > drivers/mfd/Kconfig            |   7 +++
>> > >  drivers/mfd/Makefile           |   1 +
>> > >  drivers/mfd/syscon-i2c.c       | 104
>> > > +++++++++++++++++++++++++++++++++
>> > >  include/linux/mfd/syscon-i2c.h |  36 ++++++++++++
>> > >  4 files changed, 148 insertions(+)
>> > >  create mode 100644 drivers/mfd/syscon-i2c.c
>> > >  create mode 100644 include/linux/mfd/syscon-i2c.h
>> > >
>> >
>> > [..]
>> >
>> > > +static struct regmap *syscon_i2c_get_regmap(struct i2c_client
>> > > *client,
>> > > +					    struct regmap_config *regmap_config)
>> > > +{
>> > > +	struct device *dev = &client->dev;
>> > > +	struct syscon *entry, *syscon = NULL;
>> > > +
>> > > +	spin_lock(&syscon_i2c_list_slock);
>> > > +
>> > > +	list_for_each_entry(entry, &syscon_i2c_list, list)
>> > > +		if (entry->dev == dev) {
>> > > +			syscon = entry;
>> > > +			break;
>> > > +		}
>> > > +
>> > > +	spin_unlock(&syscon_i2c_list_slock);
>> > > +
>> > > +	if (!syscon)
>> > > +		syscon = syscon_i2c_register(client, regmap_config);
>> > > +
>> > > +	if (IS_ERR(syscon))
>> > > +		return ERR_CAST(syscon);
>> > > +
>> > > +	return syscon->regmap;
>> > > +}
>> > > +
>> > > +struct regmap *syscon_i2c_to_regmap_config(struct i2c_client *client,
>> > > +					   struct regmap_config *regmap_config)
>> > > +{
>> > > +	return syscon_i2c_get_regmap(client, regmap_config);
>> > > +}
>> > > +EXPORT_SYMBOL_GPL(syscon_i2c_to_regmap_config);
>> > > +
>> > > +struct regmap *syscon_i2c_to_regmap(struct i2c_client *client)
>> > > +{
>> > > +	return syscon_i2c_get_regmap(client, &syscon_i2c_regmap_config);
>> > > +}
>> > > +EXPORT_SYMBOL_GPL(syscon_i2c_to_regmap);
>> >
>> > What do you think about
>> >
>> > struct regmap *syscon_i2c_to_regmap(struct device *dev)
>> > {
>> > 	struct i2c_client *client = i2c_verify_client(dev);
>> >
>> > 	if (!client)
>> > 		return ERR_PTR(-EINVAL);
>> >
>> > 	return syscon_i2c_get_regmap(client, &syscon_i2c_regmap_config);
>> > }
>> >
>> > Or even move it to syscon_i2c_get_regmap().
>> >
>> > This way, (a) a driver doesn't have to use "#include <linux/i2c.h>" just
>> > to call to_i2c_client() (or i2c_verify_client()) and (b) you won't do it
>> > all over again in all sub drivers.
>> >
>> > So you could just do a
>> >   regmap = syscon_i2c_to_regmap(pdev->dev.parent);
>> >
>> > I've also noticed that the mmio syscon uses device_node as parameter.
>> > What
>> > was the reason to divert from that? Just curious.
>> 
>> How is this supposed to be used?
>> 
>> I had something like the following in mind:
>> 
>> &i2c {
>>   cpld@4a {
>>     compatible = "simple-mfd";
>>     reg = <0x4a>;
>> 
>>     gpio@4 {
>>       compatible = "vendor,gpio";
>>       reg = <0x4>;
>>     };
>>   };
>> };
> 
> Yes, that was the idea.
> 
>> But I think the childen are not enumerated if its an I2C device. And
>> the actual i2c driver is also missing.
> 
> What do you mean?  Can you elaborate?

There is no i2c_driver instance who would create the regmap. If I'm
reading the I2C code correctly, it won't probe any i2c device of a
bus if there is no i2c_driver with an associated .probe() or
.probe_new(). And even if it is probed, its subnodes won't be
enumerated; the "simple-mfd" code only works for MMIO busses, right?
Or I'm getting something really wrong here..

-michael